SH(2) System Calls Manual SH(2)

Sh - module interface to the shell

include "sh.m";
sh := load Sh Sh->PATH;
Context, Listnode: import sh;
system:           fn(drawctxt: ref Draw->Context, cmd: string): string;
run:              fn(drawctxt: ref Draw->Context, argv: list of string): string;
parse:            fn(s: string): (ref Cmd, string);
cmd2string:       fn(c: ref Cmd): string;
list2stringlist:  fn(nl: list of ref Listnode): list of string;
stringlist2list:  fn(sl: list of string): list of ref Listnode;
Context: adt {


        new:            fn(drawcontext: ref Draw->Context): ref Context;


        get:            fn(c: self ref Context,


                           name: string): list of ref Listnode;


        set:            fn(c: self ref Context,


                           name: string,


                           value: list of ref Listnode);


        setlocal:       fn(c: self ref Context,


                           name: string,


                           value: list of ref Listnode);


        envlist:        fn(c: self ref Context):
			              list of (string, list of ref Listnode);


        push, pop:      fn(c: self ref Context);


        copy:           fn(c: self ref Context, copyenv: int): ref Context;


        run:            fn(c: self ref Context,


                           args: list of ref Listnode,


                           last: int): string;


        addmodule:      fn(c: self ref Context, name: string,


                           mod: Shellbuiltin);


        addbuiltin:     fn(c: self ref Context, name: string,


                           mod: Shellbuiltin);


        removebuiltin:  fn(c: self ref Context, name: string,


                           mod: Shellbuiltin);


        addsbuiltin:    fn(c: self ref Context, name: string,


                           mod: Shellbuiltin);


        removesbuiltin: fn(c: self ref Context, name: string,


                           mod: Shellbuiltin);


        fail:           fn(c: self ref Context, ename, msg: string);


        options:        fn(c: self ref Context): int;


        setoptions:     fn(c: self ref Context, flags, on: int): int;
};
Listnode: adt {


        cmd:    ref Cmd;


        word:   string;
};
Cmd: adt {


        # private data
};
Shellbuiltin: module {


        initbuiltin:    fn(ctxt: ref Context, sh: Sh): string;


        whatis:         fn(ctxt: ref Sh->Context, sh: Sh,


                           name: string, wtype: int): string;


       runbuiltin:     fn(ctxt: ref Context, sh: Sh,


                           cmd: list of ref Listnode,


                           last: int): string;


        runsbuiltin:    fn(ctxt: ref Context, sh: Sh,


                           cmd: list of ref Listnode): list of ref Listnode;


        getself:        fn(): Shellbuiltin;
};

Sh is a command-line interpreter and a scripting language; it also presents a module interface to allow Limbo modules to access its functionality at a lower level. The Sh module can be used in several different ways. At the simplest level, it can be run as a command-line program; for details of this, see sh (1). The simplest access at the Limbo level is through the system function, which given a draw Context (see draw-context (2)) and a string executes the sh command contained in s and returns its result. It catches any exceptions raised by the command. Almost as simple is run, which runs argv as a command, taking the first word as the command to be executed (it can be a braced block) and giving the rest as arguments, catching any exceptions raised.

Although program arguments are passed to external programs as lists of strings, at the Sh module level, an argument list is held as a list of ref Listnode. A Listnode holds either a simple string, or a braced block that has been parsed by the shell. Sometimes it can hold both; in this case the string and the block both represent the same thing. Parse converts from a string to a Cmd (a braced block). It returns a tuple (cmd, error) where cmd holds the parsed block, and error is non-empty if an error has occurred doing so. Cmd2string performs the opposite conversion; it returns a string that when parsed will yield the same command block it was passed. The utility functions List2stringlist and stringlist2list convert from and to a list of ref Listnode to or from a list of string respectively.

A Context holds all the state information needed by a currently running sh process; this adt holds current values of environment variables and a list of currently loaded modules and builtin commands. It is specific to the process within which it was created. If it is desired to run sh commands in a newly spawned process, a new Context must be created, or a copy of an existing Context made (making sure to synchronise access until the copy has been made).

New creates a new context. Drawcontext represents the current graphics context within which sh commands will be run (see draw-context (2)).
Get retrieves the value of environment variable name from ctxt. It is retrieved from the innermost scope in which a value for name has been set.
Set sets the value of environment variable name in ctxt to value. It is set in the innermost scope in which a value for name has been set, or the outermost level if it has not been set.
Similar to set() except that the value is set in the innermost scope that has been pushed.
Envlist retrieves the list of all the environment variables currently in scope, and their values. It returns a list of (name, value) tuples, where name is the name of the variable and value is its value.
Push creates a new innermost environment variable scope.
Pop discards the current innermost scope, losing the values of all variables that have been defined there. It is an error to pop a context that has not been pushed. Care must be taken to ensure that a push is always matched by a pop. In particular, exceptions should be caught, the context popped, and the exception re-raised.
The shell's Context is associated with a particular process; copy returns a copy of ctxt associated with the current process. If copyenv is non-zero, the whole environment will be copied - this should be set if the new process is to run asynchronously - i.e. if there is a chance that there might be two processes accessing the context in parallel. It is an error to copy a context if not within a new process.
Run executes a sh command. Last should be non-zero if this is the last time that run will be called, so sh does not have to spawn a new process in order to hide file redirection side-effects.
Addmodule adds the Shellbuiltin module mod to its list of builtin modules. The module will be initialised as described in ``Builtin modules'', below.
Addbuiltin may be called by a module that has previously been loaded by addmodule or by the load sh command to add a new builtin command to the shell. Any subsequent invocation of name within ctxt will result in a call of runbuiltin() to mod. Any attempt to redefine the command ``builtin'' will be ignored.
Removebuiltin removes name from the list of builtin commands in ctxt. If name had not previously been defined by mod, or had subsequently been replaced, then this function does nothing.
Addsbuiltin may be called by a module that has previously been loaded by addmodule or by the load sh command to add a new builtin substitution operator to the shell. Any subsequent invocation of ${name} within ctxt will result in a call of runsbuiltin() to mod.
Removesbuiltin removes name from the list of builtin substitution operators in ctxt. If name had not previously been defined by mod, or had subsequently been replaced, then this function does nothing.
Fail prints msg to the standard error if message printing is currently enabled, and raises the exception fail:ename.
Options returns a bitmask of the options currently enabled in ctxt. The bits are defined by constants declared within Context. They include:
Sh is currently being run from an interactive command-line.
Message printing is currently enabled.
Commands are printed to standard error as they are executed.
An exception will be raised when the first simple command returns an error status.

Options are defined in the innermost scope of ctxt and will be lost when it is popped.

Setoptions sets the specified flags within ctxt. Flags is a bitmask of options, as described in options, above. If on is non-zero, the specified bits will be set; otherwise they will be reset. Setoptions returns the previously set options bitmask.

Shellbuiltin specifies the interface to a loadable sh builtin module. Any Limbo module mod adhering to this interface may be loaded into the shell.

Initbuiltin is called when sh loads mod either via the load command, or via the loadmodule() function. Ctxt is the context within which the builtin has been loaded, and sh is the Sh module itself. When initbuiltin is called, mod is expected to call ctxt.addbuiltin and ctxt.addsbuiltin to define any builtin commands and builtin substitution operators that it wants. If an error occurs on initialisation, initbuiltin should return a non-nil value; this will cause the load to fail.
Runbuiltin is invoked when sh executes a command that has previously been defined as a builtin command by mod. Ctxt is the current execution context (which may not be the original context passed to initbuiltin()), sh is the running Sh module, and cmd is the command to be executed. Last is true if this is the last command to be executed in the current process; it can be passed to ctxt.run() as appropriate. The name of the command can be found in (hd cmd).word. Runbuiltin returns its exit status; by convention this is the exit status of the last command executed. A non-nil exit status is usually treated as false. By convention, if an invalid set of arguments are passed to a builtin command, a usage exception is raised by calling ctxt.fail with usage and an explanatory usage message as arguments.
Similar to runbuiltin, runsbuiltin is called when sh encounters a builtin substitution operator that has previously been defined by mod. It returns the list of values that will be substituted in place of the operator.
Getself should return the Shellbuiltin module handle for mod, usually obtained by invoking load $self. N.B. it is important that the value returned by getself is the same as that passed to addbuiltin or addsbuiltin. As the Limbo load operator returns a different value each time, the value to be returned by getself() should be initialised once, during the call to initbuiltin().
Whatis is called by the shell's whatis command to query the definition of a name. Wtype gives the type of name that is being asked about; it can be BUILTIN (conventional commands), SBUILTIN (substitution builtins), or OTHER (any other names that the module defines). Return nil to get the usual default behaviour. The std module, for example, uses this feature to display the definition of a shell function correctly.

The exceptions used within sh are exactly the same as those used within Limbo, except that all exceptions generated by the shell are prefixed by the string ``fail:'', and any exception caught with the prefix fail: has its first 5 characters removed before being made available to the sh script. This adheres to the convention defined by other shells within Inferno that a process that raises an exception with a fail: prefix is just returning a non-zero exit status, and should not be left in a Broken state. It also means that the number of bytes available for the exception string is reduced by 5 (to 59). Care must therefore be taken to avoid generating an exception with a name that is too long; sh takes the pragmatic approach of truncating any exception string that is too long.

/prog/pid/wait
The file used by the shell to wait for dead child processes.

/appl/cmd/sh/sh.y

sh (1), sh-std (1), sh-expr (1), sh-tk (1)